The acoustical frequency response of an automobile audio system, measured at the listener position, is usually far from ideal. For example, a door mounted speaker which must be positioned behind a map pocket can affect the audio reproduction. It is therefore desirable to compensate for this response by imposing a correction signal upon the transducer to compensate for the acoustic aberrations due to the physical features of the interior of the vehicle and the speaker positions.
Equalization has been implemented electronically with common analog filter circuits, but these are susceptible to performance variations due to temperature and part tolerances. Due to limits in analog circuit capabilities, the previously known equalization circuits constructed according to a calculated design may only yield an approximate response correction. The design and construction of an analog equalization circuit can be time consuming, requiring several design, build and test iterations to achieve the desired response.
Another known apparatus for changing audio reproduction characteristics is disclosed in U.S. Pat. No. 5,146,507. An audio reproduction characteristics control uses parameter memory means for correcting frequency response by prestoring correction parameters, reflection characteristic parameters and sound-field characteristic parameters. However, there is no indication or suggestion as to how the parameters may be calculated or how they could be adjusted to accommodate differing environments such as automobiles having different interior dimensions and passenger accommodations. While the parameters may only be generally related to the acoustic conditions such as vehicle type, the prestored parameters can have a substantially different effectiveness in particularly different environments or vehicles. While the parameter selection means permits various prestored parameters to be employed, the parameters are unrelated to the particular environment in which the audio reproduction system may be installed.
A further known improvement in modifying frequency characteristics of an audio reproduction is incorporated in the eclipse ESQ-2000 by compensating for the acoustics of a particular vehicle interior. The apparatus permits access to a parametric equalization mode. A pink noise signal generated in the audio unit is delivered to the front and rear speaker channels. The acoustic reproduction is analyzed after being received in a sensor such as the microphone built into the front panel of the unit. The processor automatically compensates for response anomalies by providing a flat response to the generation of the pink noise. This is achieved through the use of parametric equalization filters. Each filter's center frequency can be set to one of twenty-six preset values between 63-20,000 Hz. While such a system provides additional control over the previously known units, the fixed microphone location does not provide the type of compensation parameters that would necessarily provide the most desirable sound at the location of an occupants ears within the vehicle passenger compartment. Moreover, although the unit includes a jack that accepts an external microphone, a single microphone still limits the directionality if not the listening position at which the parameters for adjusting the signal characteristics are determined. Furthermore, the only equalization response permitted is a flat response, and only over certain frequency bands, and such response may be undesirable in particular motor vehicle designs. Moreover, since the audio unit itself generates the pink noise signal, inaccuracies within the receiver section of the entertainment device will not be compensated for by the equalizing mechanism. In addition, such dash mounted apparatus provide very little visual assistance to the operator of the system except to identify the particular operational mode in which the audio reproduction device operates. In addition, the large number of controls and button layout complicates manipulation of the acoustic mechanism and the user's interactions with the unit.